1. Field of the Invention
The present invention relates to a wire bound telecommunications device to be coupled to telecommunication lines comprising a transmission circuit coupled between the telecommunications lines, which transmission circuit comprises a power supply circuit for forming a supply voltage from a telecommunications line voltage, whereby the power supply circuit comprises sensing means for sensing a telecommunications line current and adjustment means for adjusting the supply voltage, a supply voltage terminal being coupled to the sensing means. Such a wire bound telecommunications device can be a wire bound telephone device, a base station of a cordless telephone apparatus, a facsimile apparatus, a personal computer having a telephone card, or the like.
The present invention further relates to a power supply circuit for use in such a telecommunications device.
2. Description of Related Art
A wire bound telecommunications device of this kind is known from the Philips Application Note xe2x80x9cApplication of the TEA1093 handsfree circuitxe2x80x9d, ETT/AN93015 of 29.11.1993, pp. 1-67. On page 39 of this application note, in FIG. 20, a block diagram is given showing a part of a wire bound telephone device, a diode bridge coupled to telephone lines a/b-b/a, and a transmission circuit TEA111x coupled between the telephones lines. The transmission circuit TEA111x, described in more detail in the Philips Datasheet TEA1112, xe2x80x9cLow voltage versatile telephone transmission circuits with dialler interfacexe2x80x9d, 16.2.1996, pp. 2-8, and 15, at least partly comprises a power supply circuit for forming a supply voltage from a voltage across the telephone lines. Other components of the power supply circuit are external to the transmission circuit and/or are comprised in a handsfree circuit TEA1093 which is coupled to the transmission circuit and to the telephone lines. The circuit TEA1093 is described in more detail in the Philips Datasheet TEA1093, xe2x80x9cHands-free ICxe2x80x9d, 9.2.1996, pp. 1-10. The circuitry in the ICs TEA1112 and TEA1093 and the external circuitry can also be embodied and divided over a configuration of ICs and external circuitry in principle having the same overall functionality. The power supply circuit comprises sensing means for sensing a telephone line current in the form of a so-called slope resistor SLPE of e.g. 20 Ohms and internal circuitry in the transmission IC TEA1112. At an output terminal of the power supply circuit a voltage VBB is present for supplying a load, such as a loudspeaker, via a loudspeaker amplifier fed by the voltage VBB. The available line current is determined by the telecommunications network to which the wire bound telecommunications device is to be coupled and the length of the telecommunications line. On the one hand, with fluctuating line currents, the line voltage should not increase above a given DC current mask of a given country and should not increase above a given threshold value determined by a given IC process, but on the other hand maximum power should be available as a function of the line current so that maximum power is available for the loudspeaking function, i.e., maximum loudspeaker should be obtained at a given line current. For adjusting a voltage across the transmission IC such that a maximum line voltage is achieved, adjusting means are provided in the form of an external resistor coupled to an adjustment terminal and internal circuitry in the IC TEA1112. The internal circuitry in the TEA1112 comprises a series arrangement of diodes coupled between the sensing terminal and the adjustment terminal, and an operational amplifier controlling a bipolar transistor. Internally in the IC TEA1112, the adjustment terminal is coupled to an input of the operational amplifier of which an output is controlling the bipolar transistor of which an output terminal forms a supply voltage terminal. If the external resistor is coupled to the supply voltage terminal, the voltage across the transmission circuit decreases with a decreasing value of the external resistor, whereas, if the external resistor is coupled to the sensing terminal, the voltage across the transmission circuit increases with a decreasing value of the external resistor. The internal circuitry of the TEA1112 as described can be found in the block diagram in FIG. 1 on page 3 of said Datasheet of the TEA 1112. Adjustment using the external resistor is not optimal to achieve closest following of the DC-mask so as to obtain a maximum output power as a function of the line current. It is thus a disadvantage of the known circuit that no maximum output power as a function of the line current is obtained for powering a load such as a loudspeaker amplifier coupled to a loudspeaker with this maximum power.
A general solution to the problem how to achieve optimal matching of the power supply load line to the DC-mask is described in the yet unpublished European patent application No. 96202692.8, filed by the same applicant on Sep. 26, 1996. The general solution proposes a multiple slope power management method.
It is an object of the present invention to provide a wire bound telecommunications device of the above kind wherein the DC mask is followed in a more optimal way so as to get more output power for supplying a load.
To this end the wire bound telecommunications device according to the present invention is characterized in that the power supply circuit comprises a controllable current source coupled to an adjustment terminal of the adjustment means and that a control input of the controllable current source is coupled to the sensing means, whereby the power supply is arranged such that the adjustment means operates as current sinking or current sourcing means, respectively, so as to cause the supply voltage to increase or decrease, respectively, in accordance with a predetermined function. Herewith, within an operational area of line currents, the voltage between the supply voltage terminal and a sensing terminal, i.e., the voltage across the transmission circuit is in accordance with the predetermined function of the line current.
When the control element comprises a bipolar transistor having a resistor in its emitter base path it is achieved that the predetermined function is a linear function and when the control element comprises a diode in the emitter base path it is achieved that the predetermined function is a logarithmic function, so that advantageously different DC-masks of different countries can be tracked. In other, embodiments, other predetermined functions can easily be obtained by modifying the control element accordingly. A non-linear element can be chosen for the control element having a predetermined transfer characteristic. When the controllable current source is a bipolar transistor and a control resistor is coupled between the control input and a first main electrode of the bipolar transistor, and wherein the bipolar transistor starts conducting if a voltage across the control resistor representative of the control voltage exceeds a given threshold value, it is achieved that a start current is defined so that the circuit can easily be adapted to the demands of various countries. When a current limiting resistor is coupled between an output of the controllable current source and the adjustment terminal; and after xe2x80x9cprotectionxe2x80x9d insert is achieved; maximum voltage protection. When a constant voltage resistor is coupled between the adjustment terminal and an output terminal of the sensing means, it is achieved that the voltage between the adjustment terminal and the sensing terminal is constant. This is achieved by said resistor and the described internal circuitry in the TEA1112. When the supply voltage is fed to a voltage stabilizing circuit of which an output forms the supply for a load, an output voltage is achieved in a way similar to the way disclosed on page 39 of said application note ETT/AN93015. When the voltage across the control element is fed to a further controllable current source for controlling current sinking or sourcing of a reference terminal of the voltage stabilizing circuit, the voltage across the control element is advantageously used to control a further controllable current source, e.g., in the handsfree IC TEA1093. Herewith tracking of the unstabilised supply output voltage and the stabilized supply output voltage is made the same while using simple electronic means. Namely, most of the circuitry is used in common.